Better than it seems. There are plenty of black cards that are just as powerful in the graveyard as they are in your hand. Try using this with Shambling Remains or any card that has unearth. Put rise from the grave in your deck to spring out a Zombie Prince of Thralls ASAP. Maybe discard your own Progenitus and watch him jump back into your library.

very useful if u wanna unearth half your graveyard on your final turn when its all or nothing.
unearth this guy, make your opponent discard a card that could potentially harm your hard fought victory, then go in ftw with all your little zombies :D
also works quite well with grixis slavedriver

I use this guy in my zombie horde deck. It's really good for it. It means that, even if you attack the zombie deck to try to reduce the amount of bad things you have to face, you will still have to face the discard. It's also pretty much uncounterable discard so even heavy control decks will lose some of their control. Another thing about it is that it discourages wrath effects. You kill all the zombies, but your whole team loses a card.